I need the walkthrough to activate the love relationship (incest) with Evelyn please
If you tell her how you feel without proper preparation, she will leave town forever, and the game becomes unwinnable.
To romance her, you need to remove an inhibition she has, which is linked to her dark secret. You MUST remove the inhibition BEFORE you talk to her about your feelings. You learn of the existence of the inhibition while revealing one of her lesser secrets, but it can only be removed after revealing her dark secret.
You get an inkling of the nature of her dark secret when you spend time eating with her for a second time. After that, it's a matter of following up on that one clue, first in the family records office (primrose will tell you where that is), then with Aldred.
In the family records office, you have to solve a puzzle - you're looking for a discrepancy in dates.
Once you have revealed her dark secret and removed the inhibiton, you can tell her how you feel without driving her to leave town.
Once you tell her how you feel without driving her away, if she likes you well enough, she invites you into her bed permanently and you become a couple. Spending time with her in the afternoons and telling Thaddeus to leave her alone both help make her like you better.